Starting in about 1985, oilfield service companies began using retrievable “MWD” (Measurement While Drilling) systems in downhole subterranean drilling environments. Such MWD systems typically provide borehole sensor electronics and mud pulse transmitters to transmit downhole numerical data in “real time” to the earth's surface via mud pulse telemetry.
Conventional designs of mud pulse transmitters (“pulsers”) in MWD systems may include a servo valve (or “pilot valve”) to control a larger main valve. For example, U.S. Pat. No. 6,016,288 (“the '288 patent”) discloses a pulser in which a battery powered on-board DC electric motor (“servo motor”) is used to operate a servo valve. The servo valve in turn adjusts internal tool fluid pressures to cause operation of a main valve (or “transmitter valve”) to substantially reduce mud flow to a drill bit, thereby creating a positive pressure surge detectable at the surface. De-energizing the servo motor results in readjustment of internal fluid pressures, causing the main valve to reopen, thereby terminating the positive pressure surge. Enablement and termination of a positive pressure surge creates a positive pressure pulse detectable at the surface. Streams of pressure pulses may be encoded to transmit data.
The servo motor in older designs such as described in the '288 patent typically rotates in one direction only, responsive to activating pulses of DC voltage. FIG. 2A in the '288 patent illustrates the disclosed assembly in a default resting position, with the servo motor inactive and the servo valve closed. FIG. 2B in the '288 patent illustrates the disclosed assembly after the servo motor has been energized to open the servo valve to its fully open position. Controls associated with the servo motor detect when the servo valve is fully open and cause the servo motor to shut off. Spring bias in the disclosed assembly, assisted by internal differential mud pressure, cause the servo valve to close again as the disclosed assembly returns to the resting position per FIG. 2A.
More recent designs of servo-driven mud pulsers have configured the servo motor to drive both the opening and the closing of the servo valve. The servo motors in these designs are thus disposed to rotate in both directions. The improved mud pulser of the instant disclosure is such a design. Controls associated with the servo motor detect when the servo valve is fully open and fully closed, usually by detecting a current spike in the servo motor when the servo valve reaches a fully open or fully closed position and can travel no further in that direction. Detection of the current spike causes the servo motor to change direction of rotation. This sequence is depicted generally in FIG. 10 and will be described in more detail further on in this disclosure.
Compensator
Pulsers according to any of the above-described designs typically collocate the servo motor and servo valve in a servo assembly. The servo assembly thus has both electrical and mechanical components, functioning together to open and close the servo valve. The orifice in the servo valve must allow drilling fluid to flow through its opening, since the fluid serves as the hydraulic medium by which the servo assembly controls operation of the transmitter valve. However, the servo motor and other electrical components of the servo assembly must also be sealed off from the drilling fluid in order to prevent the fluid (which is typically electrically conductive) from adversely affecting the operation of the servo motor. In particular, the drilling fluid should be prevented from contacting and shorting out the electrically-powered actuator in the servo assembly. (Typically the actuator includes a lead screw whose rotation in either direction by the servo motor causes corresponding extension and retraction of a pulser shaft into and out of the orifice in the servo valve). The sealed off area for electrical components is typically termed the “oil chamber” because once sealed, it is preferably filled with an electrically non-conductive, incompressible fluid, such as oil.
Oil chamber designs must be able to compensate for significant changes in external pressure and temperature as the drill string bores into the Earth. As the string bores deeper, the ambient drilling fluid pressure and temperature around the oil chamber will increase. As the ambient drilling fluid pressure increases, the oil chamber will tend to experience volume decrease even though the oil in the chamber is deemed “incompressible”. (It will be appreciated that the term “incompressible” is a term of art rather than an absolute parameter, allowing for some small degree of compressibility). Moreover, as the ambient drilling fluid temperature increases, the oil in the chamber will tend to expand. Failure to compensate for these volumetric changes inside the oil chamber can create a pressure differential across the oil chamber seal between the oil inside the chamber and the drilling fluid outside the chamber. Such a pressure differential results in the actuator having to work harder and thus potentially drawing more current than for which it is designed. This can cause a significant decrease in life of the actuator and ultimately the servo motor. The pressure differential can become so great that the actuator can no longer overcome it, causing the actuator to lock up. The pulser will cease to function until the pressure differential is relieved.
Pressure compensation in the oil chambers described above thus becomes an important design concern in developing robust and dependable mud pulsers. There are at least two currently known pressure compensator assembly designs, each of which has its drawbacks. The first (and most common) prior art design is a compensating piston, as shown generally on FIG. 1. On FIG. 1, and responsive to an actuator (not illustrated), a pulser shaft 101 reciprocates into (broken lines) and out of (unbroken lines) an orifice 102 in servo valve 103. Compensating piston 104 is disposed to move within sleeve 108. Pulser shaft 101 reciprocates through an opening in the center of compensating piston 104, and the reciprocation of pulser shaft 101 is independent of any movement of compensating piston 104 within sleeve 108. Compensating piston 104 separates the oil chamber 105 from the drilling fluid 106. Dynamic seals (such as o-rings) 107A and 107B respectively maintain separation of oil chamber 105 and drilling fluid 106 by sealing the interfaces between compensating piston 104 and sleeve 108, and between compensating piston 104 and pulser shaft 101. As oil in the oil chamber 105 wants to expand due to temperature or compress due to pressure, compensating piston 104 will move accordingly in sleeve 108, allowing the oil volume to change as needed.
The drawback with the compensator design per FIG. 1 is that solids in the drilling fluid 106 on the environment side of compensating piston 104 often cause the piston to get stuck in the sleeve 108. Once stuck, compensating piston 104 loses its ability to compensate. As noted above, failure to compensate the oil chamber 105 generally will allow a pressure differential to build between the oil in the chamber and the ambient drilling fluid, eventually causing the actuator to lock up and the pulser to cease functioning. Further, solids around the compensating piston 104 in the prior art design of FIG. 1 may cause seals 107A and 107B to deteriorate, in turn causing leakage of drilling fluid 106 around the compensator piston 104 into the oil chamber 105. The oil will now become electrically conductive, potentially causing the actuator to short out.
A second known (prior art) pressure compensator assembly design for oil chambers is shown generally on FIGS. 2A and 2B. This second design provides a bladder 209 instead of dynamic seals 107A and 107B on FIG. 1 to separate oil in the oil chamber (205 on FIGS. 2A and 2B) from drilling fluid (206 on FIGS. 2A and 2B).
Referring to FIGS. 2A and 2B, and responsive to an actuator (actuator housing 211 partially illustrated), a pulser shaft 201 reciprocates into (FIG. 2A) and out of (FIG. 2B) an orifice 202 in servo valve 203. Pulser shaft 201 is rigidly connected to end cap 212. Seal rings 210 sealingly secure bladder 209 to actuator housing 211 at one end of bladder 209, and to end cap 212 at the other end of bladder 209. As noted, bladder 209 separates oil in the oil chamber 205 from drilling fluid 206. Bladder 209 comprises a deformable material (typically a rubber) that inflates or deflates in response to changes in oil volume in oil chamber 205. Bladder 209 also “accordions” back and forth as servo shaft 201 retracts from and extends into orifice 202.
The drawback with the compensator design per FIGS. 2A and 2B is that in order for the bladder 209 to accordion back and forth without tearing, it must be very thin. Thin rubber is prone to cyclic wear and rupture, particularly at the “corners” of the accordion. Further, the washing of solids in the drilling fluid flow past the bladder can also cause wear and rupture. When the bladder does rupture, the electrically-conductive drilling fluid floods the oil chamber, shorting out the actuator and other electrical parts of the servo assembly.
There is therefore a need in the art for a pulser design that includes an oil chamber pressure compensator assembly that addresses the drawbacks of existing designs. There is a need in the art for more robust, dependable, long-life pressure compensation in oil chambers in servo-driven pulsers.
Dampening of Concussive Spikes from Servo Motor Stalls
As described generally above, more recent designs of servo-driven mud pulsers have configured the servo motor to drive both the opening and the closing of the servo valve by rotating the servo motor in both directions. As shown on FIG. 10, a detectable current spike in the DC supply to the servo motor occurs when the servo valve reaches a fully open or fully closed position and can travel no further in that direction. Detection of the current spike causes the servo motor to change direction of rotation.
A problem with this design occurs, however, when the servo valve reaches a fully open or fully closed position. The servo motor stalls momentarily until the drive current is switched and the servo motor rotates in the opposite direction. The stalling effect creates and transmits a reactive energy in the form of a concussive spike back through the servo assembly. If left unchecked this reactive energy can be transmitted through to the servo motor drive shaft and cause damage to the servo motor. In some cases, the reactive energy may jam the motor, even momentarily. Further, if the frictional force created by this jam is too great, the servo motor may not be able to release when trying to turn the opposite direction. This will cause a pulsing failure.
Some prior art designs remediate reactive energy from servo motor stalls by placing a small retaining ring feature on the servo motor drive shaft. The retaining ring feature intervenes to dampen reactive energy in the servo assembly from being transmitted back into the servo motor, and particularly into the planetary gearhead within the motor. In most cases, however, this retaining ring feature is inadequate. Being interposed between the servo motor drive shaft and the servo motor itself, the retaining ring is necessarily small and light so as not to affect torque delivered by the servo motor in normal operations. Over time, the retaining ring often proves not to be strong enough to withstand the repetitive reactive and concussive forces created each time the servo valve reaches a fully open or fully closed position. The retaining ring fatigues over time until failure.
There is therefore a need in the art for a pulser design that includes an improvement in the linkage between the servo assembly and the servo motor, in order to provide more robust dampening of the reactive energy generated in the servo assembly when the servo motor stalls to change direction.
Dampening of Torsion Spikes Created by Stick-Slip
“Stick-slip” is well understood term in subterranean drilling. The term refers to torsional vibration that arises from cyclical acceleration and deceleration of rotation of the bit, bottom hole assembly (BHA), and/or drill string during normal drilling operations. Stick-slip is particularly common when a selected bit is too aggressive for the formation, when a BHA is over-stabilized or its stabilizers are over-gauge, or when the frictional resistance of contact between the wellbore wall and the drill string interacts with the rotation of the drill string.
In the case of friction between the wellbore wall and the drill string, it will be understood that the drill string and bit both normally rotate in the clockwise direction when facing downhole, responsive to torque provided by a top drive and mud motor respectively. Contact between the drill string and the wellbore wall (whether casing or formation) thus imparts a corresponding counterclockwise friction force against the drill string and BHA components. A “micro-stall” occurs whenever the wellbore's counteracting friction force exceeds the local torque or rotational momentum of the drill string in frictional contact with the wellbore. A micro-stall may be only momentary or can last up to a minute. The result, however, is that torque builds up in the local drill string while the drill string is “stuck”, until there is sufficient torque to overcome the frictional force causing the “stick”. At that point, the drill string will release, or “slip”. Such release events may be violent, often involving bursts of high rotational speed to normalize the torque and torsional deflection along a length of drill string. These release events create torsion spikes in the drill string that can be received in areas of the BHA containing sensitive and fragile MWD equipment. Exposure, and particularly prolonged exposure to these torsion spikes can damage the MWD equipment.
Servo-driven mud pulser designs such as described generally in this disclosure work closely with MWD equipment. Streams of longitudinal pulses created by the pulser in the drilling fluid (or “mud”) are conventionally encoded to transmit data between the earth's surface and MWD equipment operating downhole. As a result, MWD equipment is typically located immediately above the mud pulser unit (i.e. nearer the surface). The MWD equipment and the pulser are typically collocated in the BHA, above the bit.
It would therefore be useful for a pulser design to include an improvement configured to protect the associated MWD equipment by dampening torsion spikes from stick-slip events occurring elsewhere on the drill string. Such an improvement would be particularly useful in dampening torsion spikes originating near the pulser and MWD equipment collocated in the BHA.